If she pleads with the 18 million Americans who voted for her to bury the hatchet and get behind Barack Obama, Democrats may regain the White House. If she signals the slightest ambivalence, enough of her supporters may stay home – or vote for GOP rival John McCain – to cost Democrats the race.

For Senator Obama, the stakes couldn’t be higher. Voter surveys show a tightening race with Senator McCain. In an NBC News/Wall Street Journal poll last week, just one of two voters who supported Senator Clinton in the primaries say they now back Obama.

The McCain campaign sought to exploit the party’s internal rifts Sunday with a TV ad questioning Obama’s choice of Sen. Joseph Biden as vice president. “She won millions of votes but isn’t on his ticket,” a female narrator intones in the ad called “Passed Over,” referring to Clinton. “Why?”

Denver is abuzz this week with die-hard Clinton supporters planning protests, rallies, and celebrations that will culminate with a march on Tuesday, the 88th anniversary of the day the 19th Amendment granted women voting rights.

“Hillary supporters are still very upset with the party, very upset with the media, very upset with the process, to the point where I think millions of them will not vote for Obama in November,” says Darragh Murphy, the founder of PUMA PAC, a pro-Clinton political action committee that claims 10,000 members and is planning a series of events here.

The protests come in a week meant to showcase party unity and follow three months of overtures from the Obama campaign. The Illinois senator gave Clinton and her husband, former president Bill Clinton, prominent speaking roles at the convention. The Democratic platform this year is expected to include language condemning “demeaning portrayals of women,” a response to women’s groups who felt that a sexist news media cost Clinton the nomination.

The Democratic National Committee (DNC) on Sunday backed Obama’s recent request to restore full convention voting rights to delegates from Michigan and Florida. Those states voted for Clinton but had been punished for holding early primaries in violation of party rules.

Perhaps most significantly, Obama agreed to put Clinton’s nomination to a roll-call vote on the convention floor Wednesday, a symbolic gesture of recognition for her hard-fought candidacy.

Clinton has said that that vote would give her supporters the emotional release they need to unify behind Obama and has squelched suggestions of any secret plans to revive her candidacy.

“I know from just what I’m hearing, that there’s incredible pent-up desire,” she said at a California fundraiser last month. “And I think that people want to feel like, ‘OK, it’s a catharsis, we’re here, we did it,’ and then everybody get behind Senator Obama.”

But Obama’s concessions are a gamble that may embolden as many disaffected voters as they placate.

“Can it heal wounds or will it simply reopen them and allow them to stay raw?” asks Steven Peterson, a political science professor at Penn StateHarrisburg. “If there are people who feel extremely upset, they may use [the roll-call vote and protests] as a vehicle for trying to show support for Clinton in ways that might hurt the party.”

Some pro-Clinton blogs have seethed with anger in recent days over reports that Obama’s vice-presidential search committee never vetted Clinton. (Obama aides responded that she had already been vetted during her eight years as first lady and over the course of the grueling nomination fight.)

But if Clinton shares that outrage, she has not let on publicly. Clinton, who was reportedly interested in the vice presidency, praised the choice of Senator Biden and has asked her supporters to rally behind Obama.

Her staff has dispatched a 40-person “whip team” to Denver to keep her delegates from turning the floor vote on her nomination into a fight. And she appears increasingly likely to release her delegates before the vote, a move that would give them cover to vote for Obama.

But like nothing else since her concession speech in June, her address Tuesday night – from her choice of words to her tone and body language – could determine how many of her fiercest supporters vote for Obama.

“It depends on the quality of her appeal to them,” says Conrad McBride, a professor emeritus of politics at the University of Colorado at Boulder. “If they think she’s not being genuine, they’re not going to move. If they think she is sincerely interested in getting Obama elected, some may come along. I doubt she will win over all of them.”

A Pew Research Center report this month found that Obama has made little headway with Clinton supporters since sealing the nomination. Some 18 percent said in August that they would vote for McCain, virtually the same as in June.

One of the largest pro-Clinton events – a march through downtown Denver Tuesday – is being organized by 18 Million Voices, a grass-roots group that takes no position on Obama. Elizabeth Fiechter, a Manhattan lawyer who founded the group in June, says the march is meant to celebrate Clinton’s historic candidacy and push women’s rights to the top of the party’s agenda.

“We’re not here to protest, and we’re not here to break the party in half,” she says. “The vast majority of our supporters feel like they’re undecided. They’re very well educated, thinking women who say, ‘We’ll watch and wait to see who appeals to us the most.’ ”

Ms. Murphy, of the more belligerent PUMA PAC, says she and others will be listening closely Tuesday for clues to Clinton’s true feelings. “She’s a master of subtlety,” Murphy says.

Even so, she says, her group is looking beyond Clinton’s defeat to the next election. A Democratic loss in November, she says, is the only way to prod party leaders to reform what some Clinton supporters see as a broken nomination system. “The only way the DNC will change is if they lose,” she says. “That’s the only power they’ve left to the voters.”

At a restaurant on the outskirts of Denver Sunday night, some 60 women gathered at a PUMA PAC-sponsored screening of a documentary called “The Audacity of Democracy.” In interviews beforehand, many said they would defy Clinton’s calls for party unity.

“Senator Clinton is under tremendous pressure from the DNC to fall in line,” said Connie Kafka, a middle-aged corporate communications writer from Shell, Wyo., who says she will be voting for a Republican, McCain, for the first time in her life. “I feel in her heart, she doesn’t believe it.”